This application claims the benefit of Japanese Application 2001-59118, filed Mar. 2, 2001, the entirety of which is incorporated herein by reference.
This application claims the benefit of Japanese Application Number 2001-59118 filed Mar. 2, 2001, the entirety of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to piezoelectric/electrostrictive film elements, and in particular to piezoelectric/electrostrictive elements employed as actuators utilizing flexural displacement and sensors for detecting fluid properties, sound pressures, minute weights, acceleration, and so on, as for example, in microphones or viscosity sensors.
2. Description of the Related Art
Piezoelectric/electrostrictive film elements have conventionally been used as actuators and various types of sensors. A piezoelectric/electrostrictive film element used as a sensor is used for measuring the properties of a fluid such as the density, concentration, and viscosity as disclosed in Japanese Patent publication No. 5-201265A. Such elements are used as sensors because there is a correlation between the amplitude of a piezoelectric oscillator and the viscosity resistance of a fluid in contact with the oscillator. A mode of vibration in a mechanical system such as the vibration of an oscillator can be converted to an equivalent circuit in an electrical system. A piezoelectric/electrostrictive oscillator vibrates in a fluid, and receives a mechanical resistance based on the viscosity resistance of the fluid.
The oscillator thereby senses the variation of an electrical constant of an equivalent electrical circuit of the piezoelectric/electrostrictive element of the oscillator. As a result, it becomes possible to measure various parameters, which include the viscosity, density, and concentration of the fluid.
Fluids that can be measured include liquids and gases and include not only liquids consisting of a single component such as water, alcohol, and oils but also fluids composed of slurries, and pastes obtained by dissolving mixing or suspending soluble or in soluble media. Electrical constants to be detected include loss factor, phase, resistance, reactance, conductance, susceptance, inductance, and capacitance. In particular, preferred electrical constants are loss factors and phase because they have a single maximum or minimum point of variation near a resonance frequency of an equivalent circuit. This makes it possible to measure not only the viscosity of a fluid but also the density and concentration of the same. For example, the concentration of sulfuric acid in an aqueous solution of sulfuric acid can be measured through the use of the above electrical constants. In addition to the use of electrical constants, the variation in resonance frequency may also be utilized as an index for sensing variations in the mode of vibration, if there are no specific problems from the standpoint of precision of measurement and durability.
A conventional piezoelectric/electrostrictive film element is also disclosed in Japanese Patent publication 6-267742A.
An auxiliary electrode 8 is formed at a position independent of a lower electrode 4 on a substrate 1 which is made of ceramic having a thin diaphragm portion 3 and thick portion 2 on the periphery thereof as shown in FIGS. 3A, 3B, and 3C. A portion of the auxiliary electrode is positioned beneath a piezoelectric/electrostrictive film 5. As a result of this configuration, it is possible to improve the reliability of the connection of an upper electrode 6 through the continuous formation of the upper electrode on the face of the auxiliary electrode 8 and the piezoelectric/electrostrictive film 5 without any breakage. In FIGS. 3A, 3B, and 3C, a fluid to be measured is introduced into a cavity 10 via through hole 9.
Further, a piezoelectric/electrostrictive film element is disclosed in Japanese Patent publication 6-260694A. As shown in FIG. 3, a piezoelectric/electrostrictive film 5 is on a lower electrode 4 and is of a size that the surrounding portion of the piezoelectric/electrostrictive film 5 extends beyond the electrode 4 such that the periphery of the piezoelectric/electrostrictive film 5 projects above the ceramic substrate 1. As a consequence, it is not necessary to precisely position the lower electrode 4 and the piezoelectric/electrostrictive film 5, and thus short circuits between the upper and lower electrodes are easily prevented. Further, an extending portion 11 of the piezoelectric/electrostrictive film 5 can manifest sufficient flexural displacement, generation and vibration because it is in an incompletely bonded state with the substrate 1. The extended portion 11 is not bonded with the substrate due to the presence of incompletely bonded portions 7A.
An incompletely bonded state means that a portion of the extended portion 11 is either partially bonded with the ceramic substrate or that a portion an unbonded region without any bonded portion is in existence. Specifically, it is defined to mean that the peeling (tear off) strength of the film 5 to substrate 1 is 0.5 kg/mm2 or less.
With respect to the formation of in unbonded state as described above, there are instances when it is necessary to have a low reactivity between the materials selected for the substrate and the piezoelectric/electrostrictive film. In this regard, it is also possible to form a dummy layer between the piezoelectric/electrostrictive film and the substrate so as to prevent their direct contact. The dummy layer is formed by a stamping method, a screen printing method or an ink jet method. The incompletely bonded portion 7A is formed when the dummy layer is subsequently dissolved. For example, the dummy layer is fabricated with combustible/removable materials, such as resin materials, which are dissolved away to form the incompletely bonded portions 7A when the piezoelectric/electrostrictive film 5 is heat treated. In the case where the piezoelectric/electrostrictive film and the upper electrode are not heat treated, the dummy layer is formed with a resin material to be dissolved in a composition such as water or organic solvents, etc. Accordingly, after the formation of either the piezoelectric/electrostrictive film alone or in conjunction with the upper electrode 6, the incompletely bonded portions 7A is formed by dissolving or removing the dummy layer.
In the case of such sensor elements whose electrical properties during vibration are detected to perform sensing, it is desirable that the electrical properties do not vary. In a conventional structure of piezoelectric/electrostrictive film elements, the electrical constants between the individual sensor elements tend to vary in both the initial phase and with the subsequent passage of time. In such cases, a bothersome fine-tuning process is required to insure the proper performance of the oscillator.
In particular, significant changes in the properties have sometimes occurred under sever conditions of use, e.g., high temperatures and humidity.
Further, cracks have sometimes occurred on a piezoelectric/electrostrictive film depending on the type of the material of the piezoelectric/electrostrictive film at edges of the thin diaphragm portion. Because the stresses originating from vibration or displacement of the thin diaphragm are likely to concentrate at this portion, it has resulted in breakage of the upper electrode to disable the element.
In conventional piezoelectric/electrostrictive film elements, an incompletely bonded portion 7B is formed over a thin diaphragm 3 and a thick portion of a substrate between a lower electrode 4 and an auxiliary electrode 8 as shown in FIG. 3B. The incompletely bonded portion 7B is in an incompletely bonded state similarly to the incompletely bonded portion 7A of the extended portion 11. Studying the conventional piezoelectric/electrostrictive film elements, we found that a time-passage variation or change in the incompletely bonded state of an incompletely bonded portion 7B is one of major factors that cause a change in a mode of variation, in the case of a sensor element or the like utilizing electrical constants during vibration. Further, such a change in a mode of variation consequently causes changes in electrical constants. Specifically, the incompletely bonded portion that has a low bonding force and low mechanical strength extends over the thin diaphragm and the thick portion. Since the thin diaphragm undergoes vibration or displacement and the thick portion is fixed, the incompletely bonded state cannot be established with high reproducibility and stability because of phenomena such as partial unbonded and micro-cracks. The micro-cracks thus generated can develop into a greater crack which can cause the piezoelectric/electrostrictive film to crack.
Further, in the structure shown in FIGS. 3A, 3B, and 3C, the piezoelectric/electrostrictive film sandwiched by the lower electrode and upper electrode continuously extends over the thin diaphragm and the thick portion. As a result, an electric field generated during a process of polarizing or driving the element will be applied to the region extending over the thin diaphragm and thick portion. It was found that this piezoelectrically activates edges of the thin diaphragm portion where stresses are likely to concentrate generate an additional stress that can result a crack.
The present invention relates to a piezoelectric/electrostrictive film element in which a lower electrode along with an auxiliary electrode, a piezoelectric/electrostrictive film, and an upper electrode are sequentially formed on a ceramic substrate having a thin diaphragm portion with thick portions on the periphery thereof. The lower electrode is formed to continuously extend over one thick portion on the periphery and the thin diaphragm portion. The auxiliary electrode is formed to continuously extend from a position of the thin diaphragm portion independent of the lower electrode to the other thick portion on the periphery. The piezoelectric/electrostrictive film is formed to extend over the lower electrode and the auxiliary electrode.
In the piezoelectric/electrostrictive film element, a piezoelectrically and electrostrictively active portion of the piezoelectric/electrostrictive film that is sandwiched between the upper electrode and the lower electrode preferably resides only in the thin diaphragm portion.
There is also provided a piezoelectric/electrostrictive film element as described above, characterized in that a bonding layer for bonding the piezoelectric/electrostrictive film and the thin diaphragm portion is provided between the lower electrode and the auxiliary electrode.